I heard thus.

At one time the Blessed One lived in the monastery offered by Anaathapindika in Jeta’s grove in Saavatthi. From there the Blessed One addressed the bhikkhus. ‘Whatever fears, misfortunes and dangers arise, they all arise from the foolish not from the wise. Thus bhikkhus, the foolish are with fears, miisfortunes and dangers.Therefore bhikkhus, to become wise inquirers, you should train thus.

When this was said, venerable Aananda asked the Blessed One, ‘Venerable sir, saying it rightly how does the wise bhikkhu become an inquirer?’

‘Aananda, when the bhikkhu becomes clever, in the elements, in the spheres, in dependent arising and in the possible and impossible, he becomes an inquirer’

‘Venerable sir, saying it rightly how is the wise bhikkhu clever in the elements?’

‘Aananda, there are eighteen elements. They are the elements of eye, forms and eye consciousness; ear, sounds, and ear consciousness; nose, scents and nose consciousness; tongue, tastes and tongue consciousness; body, touches and body consciousness; mind, ideas and mind consciousness. Aananda, these are the eighteen elements, when the bhikkhu knows and sees them, he becomes clever in the elements.